1. Field of the Invention
This invention relates to central tire inflation systems for motor vehicles and more particularly to such a system for automatically regulating pressure in the vehicle tires in response to conditions encountered during operation of the vehicle.
2. Description of the Prior Art
It is well known that traction of a vehicle's pneumatic tires may be varied by varying the inflation pressure. By decreasing the inflation pressure, the resulting increase in area of contact between the tire and its support surface (the tire footprint) produces a corresponding increase in traction, enabling operation over soft terrain such as in mud or sand, or on slippery surfaces such as snow or ice. Reduced inflation pressure results in increased rolling friction and may also result in increased tire temperatures, however. Therefore for maximum handling ability and for economic reasons it is desired to maintain tire pressure at the highest suitable pressure during operation at higher speeds and on smooth road surfaces.
Various central on board systems, generally referred to as central tire inflation systems, or CTIS, have been provided in the past for adjusting the tire inflation pressure while the vehicle is in operation. A CTIS is now considered standard equipment on many vehicles such as many types of military trucks which must operate reliably both on and off the road. The development of the CTIS for such military use is discussed in the SAE Technical Paper, Series 881829, entitled "A Giant Step Toward Soft Soil Mobility", by Fred L. Goodell, and a schematic of a CTIS now in use on many military vehicles is illustrated in that paper. Other known central tire inflation systems, and components for use in such systems, are disclosed in U.S. Pat. Nos. 3,102,573 to VanWinsen et al; 4,154,279 to Thuruta; 4,431,043, and 4,418,737 to Goodell; 4,619,303 to Brian et al; 4,640,331 and 4,754,792 to Braun et al; 4,724,879 to Schultz et al; and 4,860,579 to Beverly.
The current state of the art CTIS employs a semi-automatic control wherein the vehicle driver may actuate a preselected switch depending upon the nature or condition of the surface being driven over, and the system will automatically adjust and maintain the tire inflation pressure at a level compatible with the selected condition. In the system schematically shown in the SAE paper, the driver may select between the HIGHWAY mode in which CTIS will automatically maintain the tire inflation pressure at the recommended high pressure, for example 60 psi, for high speed highway travel, the CROSS COUNTRY mode in which the tire pressure is maintained at 35 psi for travel over open terrain, the SAND or MUD mode in which tire pressure is maintained at 25 psi, and an EMERGENCY mode in which the pressure is reduced to 12 psi. During operation, the system periodically (every 15 minutes) checks pressure in the tires and makes adjustments as necessary to maintain the pressure corresponding to the selected mode. This system also provides for more frequent pressure sensing and includes an automatic reset for returning the pressure from the EMERGENCY mode to the SAND mode after a predetermined time.
One problem with the known systems has been the requirement for driver monitoring and manual selection of the desired mode of operation. While this has generally been satisfactory for routine missions or when operating under a single condition throughout a mission, it has not been satisfactory where frequent operating condition changes are encountered or when operating under hostile conditions in that the driver could sometimes forget to shift the system from one mode to another when encountering different driving conditions. For example, if the vehicle goes from a sand condition onto a highway and commences travelling at high speed without resetting the CTIS, the low SAND or CROSS COUNTRY mode tire pressure can result in overheating and ultimate failure of the tires. Similarly, when suddenly entering soft sand or mud, momentum can be lost and the vehicle stalled for failure to promptly reduce tire pressure. It is, accordingly, a primary object of the present invention to provide an improved CTIS which will promptly and automatically select the proper operating mode or tire pressure depending upon driving conditions encountered.
Another object is to provide such a CTIS which will automatically select and maintain the desired tire inflation pressure to minimize or eliminate slippage between the vehicle tires and the supporting road surface.
Another object is to provide such a CTIS which will automatically adjust the tire inflation pressure in response to a change in slippage between the tire and the road surface.
Another object is to provide such a CTIS which senses tire slippage and automatically adjusts tire inflation pressure in response to the sensed slippage.
Another object is to provide a method of operation of a vehicle which includes monitoring the slippage between the vehicle's pneumatic tires and the road surface, and adjusting the inflation pressure as necessary to minimize such slippage while maintaining the inflation pressure at its highest pressure commensurate with safety and economic operation.
Another object is to provide such a method which utilizes a wheel slip sensor of the same type as the anti-lock braking system (ABS) which provides a signal to the CTIS on board computer which computer continuously monitors slip and automatically selects the proper inflation pressure corresponding to the vehicle speed and slip condition.